Caspofungin is the first of a new class of semi-synthetic antifungal agents belonging to the class of echinocandins. The drug is prepared by synthetic derivatisation of pneumocandin B0 which is obtained by fermentation of the fungus Glarea lozoyensis. Caspofungin inhibits the biosynthesis of β-(1,3)-D-glucan, an integral component of the fungal cell wall, and it is indicated for the treatment of invasive aspergillosis in patients who are refractory to or intolerant of other therapies, as well as empirical therapy for presumed fungal infections in febrile, neutropenic patients. Caspofungin is marketed as its diacetate salt by Merck & Co., under the trade name Cancidas®.
Caspofungin as a compound is claimed in the U.S. Pat. No. 5,378,804 issued to Merck & Co. The drug was prepared in a lengthy synthetic sequence in 0.7% overall yield from pneumocandin B0. The first two synthetic steps were the subject of the patent application EP 0 535 967 A2. Reduction of the primary amide was effected in two steps, i.e. dehydration with cyanuric chloride affording an intermediate nitrite which was reduced with sodium borohydride in the presence of cobalt(II) chloride. The aminal side-chain was introduced via substitution of 2-aminoethanethiol for the hemiaminal hydroxy function, oxidation to the sulfone, followed by substitution with 1,2-diaminoethane.
The pneumocandin B0 reduction product, compound of formula III in Scheme 1, is claimed as a compound in the U.S. Pat. No. 5,939,384.
Journet and coworkers describe an improved two-step reduction of the primary amide in a close echinocandin analogue (Journet et al., J. Org. Chem. 1999, 64, 2411-2417). When the cyanuric chloride dehydration was performed at −30° C. with careful control of the water content a very efficient reaction was obtained. The resulting nitrite function was later reduced under catalytic hydrogenation conditions in high yield.
Reduction of the primary amide with borane complexes in one step directly to the corresponding amine is disclosed in U.S. Pat. No. 5,552,521. When pneumocandin B0 was treated with an excess of borane-dimethyl sulfide complex in dry THF at 0° C. the reduction product was obtained in 43% yield. The process was improved further when 2-aminoethanethiol was replaced with thiophenol. One reaction step is omitted as the thiophenol group may be substituted with 1,2-diaminoethane directly without prior oxidation of the sulfide to the sulfone. However, thiophenol is highly malodorous and quite toxic.
The use of boronate ester protection in the synthesis of caspofungin is described in U.S. Pat. No. 5,936,062. Prior to borane reduction of the primary amide, the two vicinal diol systems are protected as the phenylboronate esters. Acidic work-up of the reaction mixture then releases reduced pneumocandin B0 in 61% yield. The patent claims the bis(phenylboronate) derivative of pneumocandin B0.
When pneumocandin B0 is reacted with phenylboronic acid prior to treatment with thiophenol under acid catalysis, certain process impurities may be minimised, cf. U.S. Pat. No. 7,214,768. Epimerisation of the benzylic position, as well as substitution of the benzylic hydroxy function, is suppressed when the benzylic alcohol is derivatised as a boronate ester.
Furthermore, in WO 2007/057141, yet another method for synthesising caspofungin is disclosed. The process relies on the two-step reduction of the primary amide to the amine via the corresponding nitrite and includes new intermediates.
Leonard and coworkers at Merck Research Laboratories provide a detailed description of the development of the caspofungin synthesis, and the presented process is said to be the actual process of manufacture, Leonard et al., J. Org. Chem. 2007, 72, 2335-2343. The phenylboronate ester protection is included in the reduction of the primary amide, as well as during the insertion of thiophenol under acid catalysis. The overall yield of caspofungin from pneumocandin B0 is reported to be 45%. The process consists of three chemical reaction steps and two chromatographic purifications.
As substitution of the hemiaminal hydroxy function directly with 1,2-diaminoethane has been found difficult (Leonard et al.), a two-step sequence has been necessary in the introduction of the ethylenediamine side-chain. According to the prevailing method disclosed in the prior art, sulphur nucleophiles such as thiophenol have been used as helping groups since they are readily substituted for the hemiaminal hydroxy function and may be expelled by a second nucleophile (i.e. 1,2-diaminoethane) directly or after oxidation. In particular, thiophenol has been successful as a helping group in the caspofungin synthesis. However, thiophenol is highly malodorous and toxic, and using it on production scale demands special equipment. Thus, a method not being dependent on the use of thiophenol is favourable both from an economical and environmental point of view.
As the prevailing methods of production of caspofungin involve many steps of synthesis and purification, and a substantial amount of material is lost during the process, there is still a need for an improved process of production of caspofungin. Furthermore, the most efficient processes available rely on the use of highly malodorous and toxic thiophenol as a helping group.